This invention relates to the accurate and precise characterization and measurement of deformation and properties of composite materials, and, more particularly, to the measurement of the properties of composites containing compressible inclusions such as voids.
Composite materials are important structural materials used in aerospace and other applications. A composite material contains two or more separate phases which retain their distinct properties within the composite. In a typical non-metallic aerospace composite material, high strength or high modulus reinforcements such as graphite, glass or Kevlar fibers are embedded in a matrix of a resin material that is much weaker and more ductile than the reinforcement. The matrix bonds together, aligns, and protects the reinforcement.
One of the most significant aspects of composite materials is that designers can tailor the properties of the material to the requirements of each individual piece of structure, so as to minimize weight and maximize performance. The composite properties are determined by the individual properties of the reinforcement and the matrix phases, and by the relative amounts of the reinforcement and the matrix present in the composite material. The properties of each batch of composite material must be known to be certain that they meet the requirements of the designer for a particular application. Since the properties of the phases of the composite remain relatively constant, it is vital that the amounts, or fractions, of the phases be known accurately for each piece of the composite material.
Although based upon sound engineering principles, today's technology of fabricating high performance structural composite laminate is not yielding the part-to-part reliability that will be required of the larger, more complex structures currently on the drawing boards. Some of the major problems in achieving this reliability are the occurrence of voids, and the lack of adequate nondestructive methods to monitor composite properties. To understand the difficulties in determining that composite properties meet design tolerances as well as in monitoring the occurrence of voids during the manufacturing process, it is necessary to understand the manner in which composite materials are manufactured.
In the manufacturing of a typical polymer-matrix composite, the fibers, usually in the form of yarns or bundles, are impregnated with the matrix resin to form a precursor material, called "prepreg". The fibers are hard and strong, while the resin is soft and flowable under pressure. Individual sheets or plies of prepreg are available in thicknesses of about 0.004-0.008 inches, with a layer of a release paper on each side of the sheet to facilitate handling. Due to air entrapment during the impregnation process, the prepreg sheets usually contain several percent voids therein, and also may have irregular surfaces.
Structures are progressively formed by stacking together or "laying up" a number of sheets of prepreg (after the release paper has been removed), which can result in further trapped air and voids due to the irregular surfaces of the sheets. The prepreg is slightly tacky to the touch, so that the lay-up is lightly bonded together and retains its shape. The lay-up is then cured by placing it into a press or an autoclave under elevated temperature and pressure. The curing induces a chemical reaction in the resin matrix material which causes it to become stronger, and also causes the adjacent sheets of prepreg to bond together. Void removal during the manufacturing process is essential to ensure structural strength and integrity of the finished parts. The desired result is a well-bonded, void-free structural part which has been tailored to have specific properties through careful selection of the constituent phases, the volume fractions, the manner in which the sheets are laid up, the curing process, and the state of cure-advancement in the starting prepreg material. Such parts are finding use in high performance aircraft, commercial aircraft, and a variety of more common applications.
In manufacturing practice it is extremely difficult to establish that voids have been removed, that the composite properties of the part meet design specifications, and that the prepreg is properly staged before curing. For example, determining the amount, or fraction, of the phases actually present in a specimen of prepreg composite material is difficult. The presence of voids further confuses the determination of phase fractions, since it is difficult to distinguish between the presence of voids and a higher fraction of a low density phase, by measurements of the externally measurable properties of the material. The same problem exists for the measurement of other properties such as viscosity which indicate state of advancement of prepreg material and the establishment of processing procedures for the composite material made from the prepreg. Manufacturers typically sell the prepreg material to volume fraction specifications, including a target volume fraction and a limit of variation, such as +/-2 percent. The users have to rely upon these figures in designing the layup and curing process, as well as to assume that the prepreg has not advanced in state of cure after its manufacture, since there is no adequate means to check these properties of the uncured prepreg.
The fabrication of prepreg to exact specifications is challenging, since the resin is flowable to permit the reinforcement to be embedded therein. The fibers are often provided in bundles or tows, which are dispersed within the resin in an ordered manner. Even though great care is taken to achieve uniformity of phase fraction and distribution, there are sometimes deviations from the specified target and from the required limits of variation.
It would be desirable for both the manufacturer and the user to be able to quickly and accurately determine the fractions of the reinforcement and matrix phases, and the amount of compressible inclusions such as voids, in the prepreg. In the past, the conventional approach for determining fractions of the phases has been by a laborious and costly destructive measurement technique. A piece of the prepreg is cut away from the rest of the material and weighed. The resin matrix is then chemically removed, leaving only the reinforcement particles. The particles are weighed, and the weight is divided by the total weight of the piece to determine weight fractions of the phases, from which volume fractions can be derived if desired. The fraction of matrix is calculated as one minus the fraction of the reinforcement.
This existing test procedure typically costs from $20 to $30 per specimen and requires 20-30 minutes to perform, with the result that relatively few specimens are tested. Because of the destructive nature and lengthy time requirements of the existing procedures, the test results cannot be used for real-time control of the manufacturing operation. As a result, entire batches of off-specification prepreg may be prepared before test results are available to know of the deviation from specification.
Accordingly, there exists a need for an approach for understanding, characterizing, and measuring the behavior of void-containing composites having a flowable matrix. In relation to the problem of most immediate interest, determination of phase fractions, such an approach would permit determination of the fractions of the phases and voids in composite materials by nondestructive measurements. Such a need is particularly acute for prepreg sheets, where a substantial fraction of internal voids is present. The present invention fulfills this need, and further provides related advantages.